The present invention relates generally to systems for optically recording information on a recording medium and more particularly to systems for optically recording information signal on a disc by means of laser light from which noise has been effectively removed.
A system using laser beams for optically recording information, such as video signals, on a recording disc has been known. Laser noise is unavoidably included in a laser light beam from a laser light source for a number of various causes. Furthermore, one kind of noise component in a laser light beam is a drift component due to various causes, such as fluctuations in the power supply for the laser light source.
This noise is generated not only in the laser light source, but also in the laser light path. For example, in order to apply information onto a laser beam, a light modulator is used to modulate the light intensity of the beam in response to the information signal. This light modulator, also, drifts due to causes such as temperature variation. The drift becomes admixed with the laser light beam passing through the light modulator.
If the information is recorded by means of a laser light containing noise, as described above, the noise will also be recorded. Consequently, there is a small S/N ratio of a signal reproduced from a disc recorded in this manner.
On the other hand, as the recording laser light beam moves progressively from the outer periphery toward the center of the recording disc, there is a decrease in the relative linear speed between the recording light beam and the recording disc. In order to maintain a constant average energy of the laser light beam, per unit area to the recording disc surface, despite this decrease in relative linear speed, it is necessary to gradually decrease the intensity of the laser light beam. A control of this kind is generally called a "radial compensation".
In carrying out radial compensation, heretofore, a variable light attenuator uses an optical filter. In this known method, however, problems have been encountered, such as a shifting of the light axis and a loss in light due to dust collecting on the optical filter.